The International Business Times has a column about the privacy of data on mobile devices such as cellphones:
This is nothing to do with recording your conversations. This is about the information on your phone: contacts, emails, calendars and so on. The police usually need a warrant to search property. For that, they have to persuade a judge they’ll probably find incriminating evidence. But if you’re arrested, they can rummage warrant-free through almost anything on or near you, like a wallet. A smartphone holds vastly more data and could reveal secrets. Courts are grappling with how much privacy a phone deserves.
Some treat phones like wallets. Others have decided it is reasonable to expect more privacy for cellphone data, and they require a judge’s say-so to search a handset. That’s more like the typical approach to personal computers. […]
Or suppose a phone is in the kitchen when cops knock. They probably need a warrant to search the house. But a California federal court has suggested that a search of the cellphone would be invalid unless the warrant specified what the police expected to find. […]
Though less novel, the law is also murky when the FBI tries to track your travels. Cellphone records can reveal where calls were placed. […]
Some things are certain. At a U.S. border, customs agents can search anything. Your private-sector boss can also rifle through your cellphone, so long as he gave it to you and warned he might monitor it. Otherwise, though, the law is that it depends. For a nation that holds its BlackBerries at least as closely as its right to privacy, that’s little comfort.
Read the full article for more information about specific cases.